The present invention relates to an apparatus for measuring the size of fine patterns formed on semiconductor wafers by an electron beam and more specifically to an electron beam measuring apparatus which prevents contamination of wafer surface and improves the precision of measurement.
In improving the measuring accuracy, conventional electron beam measuring apparatuses take such measures as increasing the electron beam current and constricting the beam.
However, as the current density of an electron beam increases, resist patterns and patterns made by some processes on the surface of a semiconductor wafer that are to be measured are damaged. For this reason, as described in the Japanese Patent Laid-Open Nos. 15604/1989 and 31008/1989, one or two rectangular apertures are used to shape the cross section of an electron beam into a rectangle, and the narrow linear electron beam thus shaped (referred to as a linear beam hereafter) is directed parallel to the edges of various patterns and sweeps across them. Secondary electrons or reflected electrons that are produced during the beam scanning are detected to measure the distance between edges.
FIGS. 2A and 2B show the relationship between such patterns and the linear beams
As shown in FIG. 2A, when the linear beam 21 sweeps laterally across a pattern 30 on a specimen, the secondary or reflected electrons from the specimen are detected to produce a reflected electron signal shown in FIG. 2B. The reflected electron signal is compared with a reference level V.sub.r to determine the width W of the pattern 30.
Similarly, the length L of the pattern 30 can be measured by deflecting the linear beam spot 21 vertically.
The accuracy of measurement improves as the width of a linear beam spot becomes smaller and the electron beam current increases. The current density of the beam can be reduced by elongating the spot. That is, when the spot is shaped into a narrower and longer spot and then moved in the width direction, the current density decreases, preventing damage on the patterns while at the same time improving the measurement accuracy.
In the case of this kind of linear beams, however, it is required to change the direction of the linear beam according to the direction of the edges of the pattern 30 to be detected.
In conventional apparatuses, since the elongate electron beam is changed in direction so that the longer side of the spot matches with the pattern edges to be measured, a slit direction adjusting mechanism is provided in a vacuum vessel to enable the slit to be changed in the direction and shape by operation from outside the vacuum vessel.
However, installation inside the vacuum vessel of the slit direction adjusting mechanism having movable parts gives rise to a problem of contaminated vacuum.